1. Field of the Invention
This invention relates to a sensor, associated circuitry, and method of operation for sensing, measuring and displaying one or more physical properties of a substance or medium such as a gas or liquid. Typically, one of the properties to be sensed and measured is temperature, although the applications of the invention are not limited to those in which temperature is one of the properties to be sensed and measured. Other properties and quantities to the sensing and measurement of which this invention is well suited are rate of flow, direction of flow, wind-chill factor, and position of a physical element or component.
The invention is especially well suited to applications in which the sensor comprises, at least in part, a positive-temperature-coefficient device (a xe2x80x9cPTC devicexe2x80x9d) formed from positive-temperature-coefficient (xe2x80x9cPTCxe2x80x9d) material together with interfaces, resistors, and terminals of ohmic material. In most applications of the invention, the PTC material of the sensor device is xe2x80x9csectoredxe2x80x9d or divided into a plurality of zones which are electrically interconnected but which may be exposed to a medium whose temperature, rate of flow, or other property varies from place to place therewithin. Different zones of the PTC material are exposed to different respective values of such properties of the medium. But all zones are operated within the self-stabilizing mode of the PTC material, which tends to hold its temperature just above the xe2x80x9ctransition temperaturexe2x80x9d or xe2x80x9cCurie pointxe2x80x9d.
A remarkable feature of the invention is that it makes possible the sensing and measurement of more than one property or quantity of a substance or medium without using more than one sensing device. The method in accordance with the invention is directed to this remarkable feature. In order to facilitate the full comprehension of this method, it will be illustrated graphically as well as in the text of the following disclosure.
2. Description of the Prior Art
The prior art includes various devices for measuring the speed of the wind and of other gas flows. Sometimes, as in the so-called xe2x80x9chot-wire anemometer,xe2x80x9d the speed of the wind, gas or other medium is evaluated by the rate at which it abstracts heat energy from an electrical resistance wire to which electrical energy is being supplied at a measurable rate. The heat power dissipated to the wind, gas or other medium is substantially equal to the electric power supplied to the resistance wire in maintaining it at a constant temperature.
The hot-wire anemometer may measure the rate of speed of the wind or other gas flow, but it is not well adapted to measurement of the direction, and hence the velocity, of the wind or other gas flow. Moreover, it is thermally inefficient and has no inherent xe2x80x9cmultiplierxe2x80x9d to impart xe2x80x9cleveragexe2x80x9d to the measurement of gas flow. Furthermore, it does not lend itself readily to combination with other elements to measure quantities such as xe2x80x9cwind-chill factor.xe2x80x9d
For a few applications, the measurement of rate of flow alone is sufficient. But for many more applications, it is necessary to ascertain both the velocity of flow and the temperature of the medium undergoing measurement. Although those quantities are combined in the wind-chill factor, the usual situation requires that they be separately determined and displayed. In the patented art, we find the following references:
U.S. Pat. No. 4,890,494xe2x80x94Osbond et al, issued on Jan. 2, 1990, discloses a probe comprising multiple toroidal PTC disks having ohmic facings and connected together in parallel. Each of the disks is not divided into zones or sectors which are differently exposed to plural aspects of the atmosphere or other gas in which the probe is immersed. Since there is no such differential treatment of various portions of the probe, there can be no source of plural signals that would permit the evaluation of two or more distinct properties of the atmosphere, such as temperature and humidity, or temperature and velocity of flow.
U.S. Pat. No. 3,604,261xe2x80x94Olin discloses a multidirectional thermal-anemometer sensor. As illustrated in FIGS. 10 through 14 of the drawings of the Olin patent, one of his sensing elements is spherical, and is divided in three dimensions like the sections of an orange in order to give a three-dimensional velocity-vector indication. But the sensing element is covered with a thin film of a metal such as segmented platinum, rather than PTC material. And the film is said to be maintained at a constant elevated temperature by control systems 29 to 32, of which no further description is given. Clearly, the temperature is not maintained constant by the self-stabilizing mode of PTC material, which is not present in the anemometer sensor of Olin.
U.S. Pat. No. 4,615,214xe2x80x94Burns shows segmented sensors disposed around the periphery of a continuous electrode in order to determine direction of the wind around the azimuth. But the sensors are piezoelectric, rather than PTC. in their operative properties.
In view of the aforementioned inadequacies of the prior art, I have provided a sensor which is new in its concept and surprising in its capabilities. In its preferred embodiment, the sensor in accordance with my invention is built around a single tablet of positive-temperature-coefficient (xe2x80x9cPTCxe2x80x9d) material to which are bonded, preferably on two sides, layers of ohmic resistive (or conductive) material. At least one of those layers of ohmic material is divided into sectors or zones. If the tablet of material is circular, the sectors may be divided along radial lines. On the other hand, if the tablet is essentially rectangular, the dividing lines may be transverse so as to produce, typically, three zones as defined by the divided layer of ohmic material.
Although the PTC material itself may be selectively reduced in cross section, it is not generally separated into disjointed pieces. And the layer of ohmic material bonded to one side of the PTC material is maintained continuous. In operation, this continuous layer, which may in turn be bonded to some other structure, is connected to a source of electric potential, preferably at a constant level.
It will be understood that the layers of ohmic material serve primarily to make electrical contact with the PTC material. In operation, the ohmic layer which is maintained continuous imparts to the xe2x80x9cbasexe2x80x9d side of the PTC material an electric potential allowing current to flow through the PTC material. The divided ohmic layer, on the other hand, is in thermal communication with the substance or medium whose properties are to be sensed. That layer need be only substantial and conductive enough to couple the respective sectors or zones of the PTC material to respective different portions of the substance or medium, even though those portions may be spatially very close to one another.
The respective parts of the divided ohmic layer are electrically connected through resistive elements to a different electric potential. Although the resistive elements may be made variable for the purpose of adjustability, their primary purpose is to provide xe2x80x9ctapping pointsxe2x80x9d for reading out voltages determined by the currents through the respective resistive elements.
On the other hand, the respective parts of the ohmic layer, or the portions of the PTC material beneath them, are coupled thermally, through contact, to respective portions of the substance or medium whose properties are to be sensed and measured. Typically, the xe2x80x9csubstance or mediumxe2x80x9d is a fluid which either flows freely over its interface with the PTC material (e.g. the wind) or flows past it in a tube, pipe, or other channel which, with its contents, is thermally closely coupled to the PTC material.
Because of the energization of the xe2x80x9cbasexe2x80x9d at a controlled level of potential, electrical currents flow from it through the respective sectors or zones of the PTC material and through their respective resistive elements to a different potential. The respective magnitudes of those currents are determined more by the xe2x80x9capparent resistancesxe2x80x9d of the PTC material than by the external resistive elements, which are small in resistivity.
The apparent resistance interposed to each such current is determined substantially, but not entirely, by the sector or zone of the PTC material which is its primary path of flow. And, for each incremental element of the PTC material, the resistance of that incremental element is determined by a xe2x80x9ccharacteristicxe2x80x9d curve which specifies its resistivity as a function of its temperature. That curve will be explained in the paragraphs which immediately follow.
For the purposes of this summary, it suffices to say that each element of the PTC material conducts a current determined by the voltage across it and by its resistance. Moreover, its resistance (because it is a PTC material) depends crucially upon its temperature. In turn, its temperature depends upon the rate at which it is transferring heat to the substance or medium which is undergoing measurement or other study, and which is thermally coupled to the PTC material.
Along a certain portion of the aforementioned characteristic curve of PTC material, located just above the temperature of the so-called xe2x80x9cCurie pointxe2x80x9d, a very small change in temperature produces a very large change in electrical resistance. Stated differently, the slope of that portion of the plot of resistance as a function of temperature is very steep. The location of that portion along the temperature scale can be adjusted by changing the ingredients of the PTC material.
Again for the purposes of this summary, the various sectors or zones of the PTC material in a sensor according to this invention can have different resistances because they have different rates of heat transfer to a thermally-coupled substance or medium which is characterized by different internal temperatures. Accordingly, the currents passing through those respective zones or sectors and their respective external resistive elements develop different voltages across those external resistive elements.
The rates of heat transfer from the respective sectors or zones of the PTC material to the gas or other medium depend upon the temperature and rate of flow of the gas or medium. For each particular sensor in a particular environment, the temperature and rate of flow of the gas or other medium past the sensor produce a characteristic pair of voltages across the external resistive elements connected in series with the respective sectors or zones of the PTC material. Surprisingly, I have found a reciprocal relationship to prevail: For every pair of voltages measured across the external resistive elements connected to the PTC material, there is only one combination of temperature and rate of flow of the gas or other medium with which the PTC material is thermally coupled. This phenomenon will be explained in the detailed specification which follows.